1. Field of Invention
This invention relates generally to methods for producing album pages for mounting photographs, and more particularly to a method to form an album page composed of two superposed panels of biaxially oriented, transparent polypropylene film joined together by parallel lines of hot melt adhesive to define channels, each of which is divided by a paper strip into front and rear compartments to accommodate photos.
2. Status of The Prior Art
The patents to Dorman, U.S. Pat. Nos. 4,452,666 and 4,471,545, disclose multi-channel jackets for accommodating microfilm strips, the jackets being formed by superposed panels of transparent Mylar polyester film which are joined together by parallel ribs of adhesive material to define channels for receiving the microfilm strips.
In the Shaine U.S. Pat. No. 4,702,026, there is disclosed a transparent jacket adapted to accommodate photos, such that the jacket functions as a photo album page. This album page is formed of superposed panels of biaxially-oriented polypropylene film which are joined together by parallel lines of pressure-sensitive adhesive.
It is noted in the Shaine patent that previously known photo album pages were formed by panels of polyvinyl chloride (PVC) permanently joined together by lines of hot-melt adhesive. However, while biaxially oriented polypropylene film has a greater clarity than PVC film and is also less expensive, it was not possible, according to Shaine, to effectively join together polypropylene panels with a hot-melt adhesive; hence one could not, prior to Shaine, exploit the advantages of polypropylene film in the context of a photo album page. In order, therefore, to create a photo album page whose panels are of polypropylene film material, Shaine uses parallel lines of pressure-sensitive adhesive to join together these panels to create channels or compartments to receive the photos.
Since the distinction that exists between a pressure-sensitive adhesive and a hot-melt adhesive is important to the present invention, this distinction must be clearly understood.
By definition, a pressure-sensitive adhesive is one that adheres or seals under the influence of pressure alone. Thus in order to get Scotch tape, a well-known brand of pressure-sensitive adhesive tape, to adhere to paper, one has only to press the tape against the paper.
A hot-melt adhesive is made of a synthetic resin which is applied in a hot molten state. Hence when two sheets are joined together by a hot-melt adhesive applied to the surfaces to be joined, it is only when this adhesive cools and hardens that a permanent bond is then created between the two sheets. If one were to thereafter peel apart the two sheets to disrupt this bond, the bond could not be restored by then pressing the two sheets together. But when a pressure-sensitive adhesive is used to join two sheets together, one can thereafter peel apart the two sheets to break the bond and then restore this bond by again pressing the two sheets together.
In the method disclosed in the Shaine patent to produce a photo album page, two webs of polypropylene film are advanced concurrently into combining rolls. Applied to the inner surface of one of these webs before it enters the combining rolls are parallel lines of pressure-sensitive adhesive in a hot, molten state. This adhesive is then rapidly cooled below its melting point so that before it enters the combining rolls where it is subjected to pressure, the adhesive is then in a solid, highly tacky state and will bring about adherence of the two polypropylene webs.
The pressure-sensitive adhesive lines in the Shaine album page define channels or compartments into which one may insert photographs or slides. In the context of a photo album page, this gives rise to two disadvantages. When in the combining rolls, the lines of pressure-sensitive adhesive are squeezed to define channels which are bordered by these lines, the squeezed lines have sticky edges or margins. Because of this, it may be difficult to insert photos into the channels having a width closely matching that of the channels, for the sticky edges of the channel borders resist insertion of the photos.
Another disadvantage is that a pressure-sensitive adhesive produces a relatively weak bond as compared to the permanent bond resulting from the use of a hot-melt adhesive. This weak bond tends to become still weaker with age. Hence an album page of the Shaine type may in time delaminate.
A factor which plays an important role in the acceptability of a photo album page or a jacket for storing microfilm strips is its archival quality; that is, the ability of the jacket or page to retain its integrity and not fall apart after a few years.
Transparent polypropylene film which is biaxially oriented is the preferred material for album pages, not only because of its great strength and high degree of clarity, but also because of its relatively low cost as compared to Mylar (polyester) and other films having similar characteristics. One must therefore take into account the archival quality of polypropylene, and in this regard the chemistry of this material must be considered.
Polypropylene is a synthetic, crystalline, thermoplastic polymer with a molecular weight of 40,000 or more. While it is not attacked by fungi or bacteria, and is resistant to most acids, it is attacked and degraded by those acids which act as oxidizing agents. If, therefore, polypropylene film panels are joined together by a pressure-sensitive or hot-melt adhesive agent which contains an acid that attacks the film, its archival quality will be impaired.
It is to be noted that polypropylene falls into a class of thermoplastic polymers derived from simple olefins known as polyolefins. This class also includes polyethylene and polybutenes.